New regulatory mechanisms of polystyrene nanoplastics on the ecological risk of zinc: Cellular oxidative injury and molecular toxicity mechanisms in soil sentinel organisms (Eisenia fetida)

Nanoplastics (NPs) and zinc (Zn), both widespread in soil environments, present considerable risks to soil biota. While NPs persist environmentally and act as vectors for heavy metals like Zn, their combined toxicity, especially in soil invertebrates, remains poorly understood. This study evaluates the individual and combined effects of Zn and NPs on earthworm coelomocytes and explores their interactions with Cu/Zn-superoxide dismutase (SOD), an antioxidant enzyme. Molecular docking revealed that NPs bind near the active site of SOD through π-cation interactions with lysine residues, further stabilized by neighboring hydrophobic amino acids. Viability assays indicated that NPs alone (20 mg/L) had negligible impact (94.54 %, p > 0.05), Zn alone (300 mg/L) reduced viability to 80.02 %, while co-exposure reduced it further to 73.16 %. Elevated levels of reactive oxygen species (ROS) and malondialdehyde (MDA) levels were elevated to 186 % and 173 % under co-exposure, alongside greater antioxidant enzyme disruption, point to synergistic toxicity. Dynamic light scattering and zeta potential (From -13 to -7 mV) analyses revealed larger particle sizes in the combined system, indicative of enhanced protein interactions. Conformational changes in SOD, such as α-helix loss and altered fluorescence, further support structural disruption. These findings demonstrate that co-exposure to NPs and Zn intensifies cellular and protein-level toxicity via integrated physical and biochemical mechanisms, providing critical insight into the ecological risks posed by such co-contaminants in soil environments.